Vacuum processing operations take place in vacuum chambers that provide near vacuum or other low-pressure environments for processing substrates. Chucks support the substrates within the processing chambers. Some such chucks merely provide a substrate support platform and rely on gravity to hold the substrates in place. Others actively secure the substrates with either mechanical or electrostatic clamps.
Some chucks are also involved with the processing of the substrates by producing electrical or magnetic fields or by regulating heat transfers to or from the substrates. Such electrical fields (e.g., bias) can be used to generate or enhance a plasma as well as to direct plasma ions impinging on the substrate. Such magnetic fields can be used to also influence the plasma or to magnetically orient films during plasma-assisted deposition or thermal anneals. Heat transfers are used to remove excess heat from the substrates produced by such processing operations or to provide a controlled amount of substrate heating for assisting such processing operations. Some operations are best performed at fixed substrate temperatures or at substrate temperatures that are adjusted throughout different stages of the operations. Plasma sputtering operations such as chemical-vapor deposition (CVD) and metal-organic chemical-vapor deposition (MOCVD) require active substrate heating, while other sputtering operations require active substrate cooling. During operations like thermal annealing, elevated temperatures actually accomplish the substrate processing.
However, controlling substrate temperatures in near vacuum or other low-pressure environments is quite difficult because heat does not transfer well between objects in such environments. For example, the conduction of heat between contiguous surfaces of a chuck body and the substrate in a low-pressure environment is slow and inefficient because actual contact on an atomic scale between the surfaces is limited to a small fraction of their common areas, and gaps that separate the remaining common areas of their surfaces prevent effective heat transfer by conduction.
Heating and cooling of substrates through radiational heat transfers are possible in low-pressure environments, particularly at elevated substrate and chuck temperatures, but radiational heat transfers are generally too slow to maintain substrates at desired processing temperatures. Below 500.degree. C., which includes most chuck-based fabrication processes, radiational heat transfers are too inefficient to regulate substrate processing temperatures.
Faster transfers are possible by pumping a gas, preferably an inert gas such as helium or argon or another gas such as nitrogen or hydrogen, between the chuck body and the substrate. Although still at much less than atmospheric pressure (e.g., 1 Torr to 20 Torr), the gas (referred to as "backside gas") sufficiently fills the small gaps between the chuck body and the substrate to support significant heat transfer through thermal conduction between them. A seal formed between the mounting surface of the chuck body and the substrate resists significant leakage of the gas into the rest of processing the chamber, which could disturb substrate processing operations.
U.S. Pat. No. 4,680,061 to Lamont, Jr. discloses chucks having heating or cooling elements for regulating substrate temperatures. One of the chucks has a ceramic heating element mounted in a cavity between a chuck body and a substrate. The heating element is mounted close to a back side of the substrate but not in contact. Argon gas is introduced into the cavity to promote heat exchanges between the heating element and the substrate. A raised rim of the chuck body on which the substrate is mounted contacts a peripheral portion of the substrate's back side forming a seal that inhibits leakage of the gas out of the cavity.
Another of Lamont, Jr.'s chucks has a chuck body that functions as a heat sink with coolant channels for removing heat from the sink. A similar cavity is formed by a raised rim in the chuck body so that the remaining heat sink is positioned close but not in contact with the back side of a substrate. Argon gas is similarly trapped within the cavity by contact between the raised rim of the chuck and the back side of the substrate. Thus, the raised rim that supports the substrate also functions as a seal for inhibiting leakage of the gas into the rest of the processing chamber.
U.S. Pat. No. 4,949,783 to Lakios et al. also discloses a chuck using gas pressure against a back side of a substrate to promote substrate cooling. A similar cavity is formed in the chuck body and surrounded by a raised rim for contacting the back side of the substrate. However, instead of merely pumping backside gas into the cavity, Lakios et al. circulate the backside gas both into and out of the cavity by establishing a gas flow. Part of the heat transfer from the substrate is due to gas-conducted heat exchanges with the chuck body, and another part of the heat transfer is due to the removal of heated gas from the cavity.
The chucks of both Lamont, Jr. and Lakios et al. include raised rims on their chuck bodies that function as both mounting surfaces and seals. Mechanical clamps press the substrates against the raised rims of their chuck bodies to tighten the seals and to reduce leakage of backside gas into their processing chambers. Lakios et al. also use an O-ring seal next to their raised rim to provide an even tighter seal for further reducing leakage. However, such O-ring seals are normally not useable for elevated substrate-temperature processing (e.g., above 200.degree. C.) because of thermal limitations of elastomer seals.
The raised rims of the prior chucks separate conductive portions of the chuck body from the substrate, which reduces efficiency of heat transfers between them. Also, some leakage of substrate backside gas can occur through the raised rims, particularly through rims made to withstand elevated temperatures during substrate heating operations. Substrate back side surface roughness can also reduce the effectiveness of the raised rim seals and lead to excessive leakage of backside gas into the processing region of the processing chamber.